Section 5.8.3
This note is in twice:
Note: If an element has multiple class attributes, their values must be
concatenated
with spaces between the values before searching for the class. As of
this time
the working group is not aware of any manner in which this situation can
be reached,
however, so this behaviour is explicitly non-normative in this
specification.
Please reconsider this note because multiple attributes of the same name
are not allowed in either XML or SGML.
Regards,
Craig Northway